Various types of wind machines including constant rotor speed and adjustable speed machines are known. Since the turbine-generator is normally directly connected to a fixed frequency electrical power grid constant speed wind turbines have predominated. Variable speed wind turbines cannot be directly connected to the grid because of their variable frequency electrical power output. This necessitates the interposition of a frequency converter between the generator and the grid to make the frequency conversion. Various techniques are well known for such frequency converters including cycloconverters and rectifier-inverters. However, such techniques have not been generally applied to the wind turbine art.
In the variable speed motor drive art, electronic speed control techniques have been developed for all types of motors using the thyristor, or silicon-controlled rectifier (SCR). It has beccme apparent to wind energy theorists that it is possible to use an adjustable speed AC drive in reverse. In other words, instead of using the adjustable speed drive as a receiver of fixed frequency AC for conversion to variable frequency AC for driving a motor, it is possible to provide the frequency converter with variable frequency AC from an AC generator for conversion to fixed frequency AC for supplying a power grid.
As known in the wind energy art, the kinetic energy available from an oncoming wind stream varies as the size of swept area, density, and cube of the wind velocity. It has been shown that no more than 59% of the energy can be extracted and the ability of any wind turbine to approach that maximum has been named the coefficient of performance, C.sub.p. Cp is related to the aerodynamic features of a given machine, particularly the tip speed ratio, which is defined as the ratio of tangential speed of the blade tip over the speed of the oncoming wind. If this ratio can be maintained at the machine's peak coefficient of performance by letting rotor speed follow wind speed, the wind turbine becomes highly efficient. In addition, variable speed wind turbines provide the opportunity for short term energy storage. I.e., wind speed transients can be integrated into speed changes.
A control strategy for such a wind turbine, based in part on electrical adjustment of generator torque was disclosed and claimed in the pending applications noted at the beginning of this application. There, generator torque is electrically adjusted to maneuver speed to obtain peak performance. This is accomplished by use of a variable speed wind turbine controller responsive to both sensed generator speed and generator power output signals. It provides a generator torque command signal to a frequency converter which in turn controls the air gap torque in the generator by controlling the level of power delivered to the power grid. The sensed power signal is provided to the controller which consults a look-up table constructed according to a desired functional relationship between generator output power and the corresponding speed necessary to obtain optimum performance or to obtain maximum throughput of energy. The controller provides a speed reference or speed command signal which is compared to the sensed generator speed signal. A difference signal indicative of the difference therebetween is integrated to provide the generator torque command signal for the frequency converter.
The air gap torque between the generator stator and rotor is effectively controlled by the generator torque command signal provided by the variable speed wind turbine controller. In effect, however, the variable speed wind turbine controller operates, at lower windspeeds, to maneuver the generator speed according to the function defining generator speed versus generator electrical power output residing in the look-up table. This function is defined in such a way that the speed maneuvers tend to cause the wind turbine to operate on the wind turbine power coefficient versus velocity ratio optimum performance curve substantially at the peak thereof, for increased efficiency.
For increased wind speeds above a selected limiting torque, the generator torque command signal holds torque essentially constant and permits the turbine rotor to operate at speeds greater than dictated by the constant velocity ratio up to a speed limit. Preferably, the constant generator torque command signal can be utilized without aerodynamic torque control in order to maximize generator output power by causing the wind turbine to operate substantially on the power coefficient versus velocity ratio optimum performance curve and increase its speed above the windspeed for the selected limiting torque until the speed limit is reached. In either case, there will be certain speeds within the operating range at which system resonances will occur. The resonances can be particularly severe at some speeds and pose potential vibration problems which are potentially most severe in consequence.